Electrically heated bedcovers, such as electric blankets, resemble conventional bedcovers but include an electrical heating element, such as a resistance wire, disposed therein to which voltage is applied to cause the element to heat and thereby provide warmth for a user.
Should the electrically heated bedcover be bunched, folded, or covered with a material having good thermal insulation quantities, an overheating condition may develop, either locally or generally throughout the bedcover. The overheating condition is detrimental to the material of the bedcover and could be dangerous to a user. With this problem in mind, electrically heated bedcovers conventionally include some type of overheat protection means for avoiding the dangerous and detrimental effects of overheating.
In one conventional arrangement, overheat protection for an electrically heated bedcover is provided by a plurality of normally closed bimetallic thermostatic switches disposed throughout the heated area of the bedcover and electrically connected in series with the electrical resistance heating element in the bedcover. Upon the occurrence of an overheating condition, the thermostats open and break the heating element circuit. Although relatively widely used, this approach is subject to a number of deficiencies. Most notably, the bimetallic thermostatic switches used for overheat protection must be of a relatively heavy-duty construction, and consequently are relatively bulky. A bedcover containing this type of thermostat exhibits a lumpy appearance which detracts from the commercial appeal of the bedcover and renders it subject to more rapid wear at the locations of the thermostat.
It has been proposed that overheat protection means in electrically heated bedcovers be constructed to avoid the aforementioned deficiencies of conventional bimetallic thermostats through the use of thermostatless sensing means for detecting the occurrence of an overheat condition. One such thermostatless sensing means employs a pair of signal wires having a temperature sensitive dielectric material therebetween. Another thermostatless sensing means, disclosed in our earlier U.S. Pat. No. 3,356,825 issued Dec. 5, 1967, uses positive temperature coefficient resistors or thermistors in the bedcover. While electrically heated bedcovers employing these types of thermostatless sensing means have been shown to be useful, such arrangements are neither compatible with existing ambient temperature responsive comfort controllers, nor are they as inexpensive to produce as the aforementioned more conventional arrangement using bimetallic thermostats.
Still another type of thermostatless overheat protection circuit is disclosed in our earlier U.S. Pat. No. 3,683,151, issued Aug. 8, 1972. This circuit employs a gate controlled semiconductor switch in a thermostatless overheat protection circuit for reducing the power delivered to the heating element in response to the occurrence of an overheating condition. While this protective circuit has been useful and successful, it utilizes a relatively expensive type of sensing wire and solid state control circuit.
With this background in mind, it is an object of the present invention to provide an electrically heated bedcover which is protected against an overheating condition and which is not subject to the deficiencies and limitations which are present in previously known electrically heated bedcovers.
It is a further object of the present invention to provide an improved overheat protective means which is applicable not only to electrically heated bedcovers, but also to various other kinds of electrical heating apparatus.